Radio access networks are rapidly becoming increasingly denser and heterogeneous as we move towards 5G. In the future, architectures of Single Radio Access Network (SRAN) will support Heterogeneous Network (HetNet) deployments in which a so called anchor node (for example, an Long Term Evolution (LTE) eNodeB) provides wide area coverage and signalling connectivity, whilst subtended small cells provide high bandwidth user plane links to users. Small cells of different Radio Access Technologies (RATs) and using different spectrum (included unlicensed spectrum) may be attached to the anchor node. In particular, in 3GPP LTE Rel-12 and currently in Rel-13, different realisations of this concept have or are being standardised. In Rel-12 Dual Connectivity (also known as LTE Multiple Stream Aggregation (MSA)) was introduced wherein both macro and small cell nodes belong to LTE, whilst in Rel-13 there are work items to standardise LTE and Wireless Local Area Network (WLAN) interworking, such as LTE-WiFi Aggregation (LWA), and License Assisted Access (LAA), respectively.
For LTE/WLAN interworking, Rel-12 specifications have introduced an Access Network Selection (ANS) mechanism for LTE/WLAN traffic steering. The User Equipment (UE) offloading decision is taken by based on assistance parameters that are provided by the cellular network. In that sense, decision thresholds with respect to signal strength/quality, load, etc. determine the condition to be met for steering traffic from and to the WLAN. Additional integration enhancements are currently considered for standardization in LTE Rel-13. These include fully network-controlled LTE/WLAN traffic steering (also known as LTE WLAN Interworking (LWI)) or even downlink LTE-WLAN Aggregation (LWA) that allows UEs to concurrently receive data from both RATs. The LWA design draws many aspects from LTE Dual Connectivity (DC). To improve the capacity of the Downlink (DL) WiFi, the Uplink (UL) WiFi Medium Access Control (MAC) control frames are sent over LTE (encapsulated by the RRC protocol), and no UL user plane is mapped to WiFi.
Regulations for accessing unlicensed spectrum require a communication device to “listen before talk”, i.e. the communication device shall assess whether the channel is already occupied by another communication device's transmission prior to transmitting itself. WiFi is today's most widely deployed technology operating in unlicensed spectrum. The WiFi system is based on the IEEE 802.11 family of standards which exploits Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to enable uncoordinated spectrum access among WiFi Access Points (APs) and WiFi stations.
Once a transmitter has been cleared to transmit by the CSMA/CA procedure, the transmitter waits for an Inter-Frame Spacing (IFS) time prior to transmitting a data frame comprising a physical layer preamble followed by a MAC Service Data Unit (MSDU) with related MAC. Upon successfully receiving a data frame requiring acknowledgment (ACK), the transmission of the ACK frame shall commence after a Short IFS (SIFS) period, without regard to the busy/idle state of the medium. The acknowledgment is further protected by an additional inter-frame space after its transmission is concluded. Other stations (such as a user or a user device) and APs shall defer accessing the channel for the whole duration of the data frame and acknowledgment transmission (including the inter-frame spacing), and at the end of this period a backoff mechanisms is started using a certain contention window. Through Clear Channel Assessment (CCA), a communication device detects whether the medium busy when the Carrier Sense/Clear Channel Assessment (CS/CCA) mechanism detects a channel busy condition. A channel busy condition is detected when the Received Signal Strength (RSS) is equal to or greater than the minimum modulation and coding rate sensitivity (i.e. −82 dBm for 20 MHz channel spacing, −85 dBm for 10 MHz channel spacing, and −88 dBm for 5 MHz channel spacing).
The CCA/CA mechanisms avoids collisions among transmissions of APs operating in the same channel by deferring the transmission of an AP whenever another AP, whose RSS exceed the CCA sensitivity threshold, starts transmitting. For downlink LWA, wherein the UL WiFi MAC control frames are sent by the UE over LTE and no UL user plane is mapped to WiFi, the CCA threshold can significantly impact the DL throughput. When two APs operating in the same unlicensed channel are sufficiently isolated (e.g., sufficiently distant from one other, or with sufficient channel attenuation in between) so that the respective RSS does not exceed the sensitivity threshold, the APs can simultaneously access the channel without colliding. On the other hand, when APs can detect each other transmission with RSS exceeding the CCA sensitivity threshold, the APs share the channel access over time using CSMA/CA, but transmission collisions may occur when backoff timer expires for both APs at the same time. During a collision the likelihood of decoding the frames correctly at the recipient STAs (stations) is low. Furthermore, after a collision the backoff window of the AP is increased so the (idle) time interval between successive frame transmissions increases. This may significantly affect the throughput/capacity of the WiFi network since WiFi APs are refrained to transmit when the channel is busy.